why doesnt my available to bet balance go up on Ante post market, help

for some reason its kept in exposure .       its the coronation cup on satorday,     everything was fine earlier this morning  , i had a very small green on most of the field, the rest a small red, but on Sunway the outsider i had a red of near £140 ,   i placed a £1 on at 140, which was matched so i could release some funds ,  as in most markets i assumed that my available to bet balance would increase, but all it does is eat up my £1 and put it in exposure,      so though my worse loss on the market is  now illinois wich costs me £20 odd quid,    there is more than £140 held in exposure.      ps hope someone understands what i mean .    something similar occured during the snooker world champs  , i know itll get sorted at the end of the race but im rather puzzled